Suspensions of cells of Klebsiella aerogenes inactivate and then proteolytically degrade glutamine synthetase. The inactivation reaction bears characteristics similar to cytochrome P-450 dependent reactions. Ascorbic acid mimics some P-450 reactions, and ascorbate does inactivate glutamine synthetase. Ascorbate-inactivated glutamine synthetase was studied to identify the altered amino acid(s). Spectral studies were consistent with the oxidation of one tyrosine/subunit to a dopa-like compound. Alkaline hydrolysis documented that the 2 tryptophan/subunit were not altered. Acid hydrolyses suggest a loss of one tyrosine and one histidine per subunit. The chemical changes identified so far can be mediated by cytochrome P-450 dependent systems. Thus, such systems may be physiologically involved in the degradation of specific proteins.